1. Field of the Invention
This invention relates to the modification of inorganic metal oxide anions, referred to as polyoxometalate anions, by the reaction with silanes having two hydrolyzable groups.
2. Background Information
Although the vast majority of metal oxides are insoluble or have a limited solution chemistry, as the chemical literature points out there is an important exception. This exception includes a class of oxides that are soluble in aqueous or nonaqueous solutions, charged and derived from polyvalent metals such as molybdenum, tungsten and less frequently vanadium, niobium or tantalum, or mixtures thereof, in their highest oxidation states. These oxides are referred to in the literature as "isopolyanions" when only a polyvalent metal and oxygen are involved and can be represented by the formula (M.sub.m O.sub.y).sup.p-. If an additional metallic or non-metallic element is present these oxides are referred to as "heteropolyanions" and can be represented by the formula (X.sub.x M.sub.m O.sub.y).sup.q-. Polyoxometalate anions fall into the latter category.
In the formulae described above, M is a polyvalent metal of the type already described, X is the "heteroatom" and can be almost any element in the periodic table, other than a noble gas, O is oxygen, x, m, and y are integers where x&lt;&lt;m, and p and q represent the charge on the anion. This charge on the anion can be calculated by multiplying the valences of X, M and O by the value of the integer x, m and y associated with that atom and adding the products together.
Although a majority of polyoxometalate anions have been formed in aqueous solution, nonaqueous syntheses have also been developed. A thorough discussion of polyoxometalate anions can be found in a text by Michael Thor Pope entitled "Heteropoly and Isopoly Oxometalates" published in 1983 by Springer-Verlag. As discussed therein, these anions may generally be isolated from solution by addition of cations, typically alkali metals, NH.sub.4.sup.+ or R.sub.4 N.sup.+, where R represents a monovalent hydrocarbon. The type of cation will affect the chemical and physical properties of the polyoxometalate anion, including its solubility, and the properties of reaction products of the polyoxometalate anion.
The chemical literature also points out that the structures of polyoxometalate anions seem to be controlled by the electrostatic and radius-ratio principles seen for extended ionic lattices. Although there are a very large number of known polyoxometalate anions, investigators have found that most of these can be characterized by relatively few structures. These structures consist of groups of MO.sub.6 octahedra surrounding XO.sub.4 tetrahedra that share edges, corners and occasionally faces with adjacent polyhedra.
The structure of a number of polyoxometalate anions is discussed in the above referenced article by Pope, in a text by M. Pope and A. Muller in Angew. Chem., (International English Edition), 30 (1991) 34-48 and in an article entitled "Heteropoly Compounds of Molybdenum and Tungsten" by G. Tsigdinos that is part of a collection entitled Topics in Current Chemistry, 76 (1978) 1-64.
The structure of a particular polyoxometalate anion, the heteropolyacid H.sub.3 PW.sub.12 O.sub.40 *6H.sub.2 O, is described by J. F. Keggin, in Nature, 131 (1933) 908-909. The acidic anion was determined to be a coordinated structure having a central PO.sub.4 tetrahedron group surrounded by 12 WO.sub.6 octahedra groups, linked together by shared oxygen atoms. Polyoxometalate anions having the polyhedron type structure are known as having a Keggin structure.
Keggin structure anions of the formula (XM.sub.12 O.sub.40).sup.n- where M is molybdenum or tungsten and X is As.sup.+5, Si.sup.+4, B.sup.+3, Ge.sup.+4, P.sup.+5, Fe.sup.+3 or Co.sup.+2 have been reported in the literature. When X is silicon and M is tungsten or molybdenum, n is 4. Under mildly basic conditions, one or more of the MO groups from a polyoxometalate anion may be removed to form a deficient or "lacunary" structure. The structure resulting from removal of one MO group can be represented by the formula (XM.sub.11 O.sub.39).sup.n-. In the example given above when X is silicon and M is tungsten or molybdenum, n is 8. The vacancy left by the departing group can be filled with other atoms or groups.
Another common structure of polyoxometalate anions is referred to as a Dawson structure and is represented by the formula (X.sub.2 M.sub.18 O.sub.62).sup.n-. The heteroatom represented by X in this structure is P.sup.5+ or As.sup.5+. A "lacunary" Dawson structure can also be formed by the removal of one or more MO groups. When one MO group is removed (which is the most common case) the "lacunary" Dawson structure is represented by the formula (X.sub.2 M.sub.17 O.sub.61).sup.n-.
Reactions of the lacunary polyoxometalate anion W.sub.11 SiO.sub.39.sup.8- with RSiCl.sub.3 where R is C.sub.2 H.sub.5, C.sub.6 H.sub.5, NC(CH.sub.2).sub.3 or C.sub.3 H.sub.5 in an unbuffered aqueous solution have been reported by Knoth, J. Am. Chem. Soc., 1979, 101:3, 759-760. Knoth determined that the WO.sup.4+ unit required for a complete or "non-lacunary" structure was replaced with (RSi).sub.2 O.sup.4+ so the anion product corresponded to the composition (RSi).sub.2 W.sub.11 SiO.sub.40.sup.4-. The structure Knoth postulated for this product, which was later confirmed, required that an oxygen atom bridge the two silicon groups that had been added to the polyoxometalate anion reactant.
P. Judenstein, et al., J. Chem. Soc., Dalton Trans., (1991) 1991-1997, reported the synthesis in water and acetonitrile of polyoxometalate salts or acids having the general formula [R].sub.4 [SiW.sub.11 O.sub.40 (SiR'').sub.2 ] where R=H.sup.+, K.sup.+, or NR'.sub.4.sup.+, R'=methyl or butyl and R''=ethyl, vinyl, phenyl or decyl. The experiments described the reaction of W.sub.11 SiO.sub.39.sup.8- with various R''SiX.sub.3 where X represented chlorine or ethoxy and R'' was as described above. The resulting structure of the reaction products was determined to be as described by Knoth such that an oxygen atom bridged the two silicon groups that were added.
Judenstein, Chem. Mater. (1992) 4, 4-7, describes the synthesis of negatively charged macromolecules by incorporating organically functionalized polyoxometalate anions in an organic polymeric backbone. The organically modified polyoxometalate anion was obtained by reacting W.sub.11 SiO.sub.39.sup.8- with RSiCl.sub.3 where R was vinyl, allyl or methacryl, or RSi(OEt).sub.3 where R was styryl and the reaction products obtained therefrom were then reacted further using a free radical polymerization.
Ammari, et al. New. J. Chem. (1991) 15, 607-608, describes the reaction of the trivacant W.sub.9 SiO.sub.34.sup.10- with RSiCl.sub.3 where R was alkyl or aryl in dry acetonitrile.
Copending U.S. application Ser. No. 08/172,787 now U.S. Pat. No. 5,391,638, provides organosiloxane compounds containing silicon-bonded polyoxometalate structures that are present as pendant groups and methods for their preparation. These methods include reacting a polysiloxane containing two or three hydrolyzable groups on the terminal silicon with a lacunary polyoxometalate anion and reacting a polyorganosiloxane containing at least one silicon bonded hydrogen atom with the product of a lacunary polyoxometalate anion and a silane having two or three hydrolyzable groups.
The materials found in the prior art have uses in coatings; in electronics, for example as sensors; as magnetic/electric storage devices; as catalysts; as ion exchange membranes in chromatography and as self reinforced elastomers.
One objective of this invention is to prepare silane modified polyoxometalate anions using silanes having two hydrolyzable groups. Another objective of this invention is the preparation of materials which can potentially be used to catalyze stereospecific reactions or be used as building blocks for organic-silicone-inorganic polymers. These materials could also potentially be used as additives in polymers to perform specific functions, for example, electrical conductivity, photochromicity or electrochromicity.